1. Field of the Invention
The present invention relates to seismic exploration, more particularly, it relates to methods for driving a continuous wave seismic source, a vibrator, with a signal having a component effective to significantly reduce harmonic distortion in the output of the vibrator.
2. Discription of the Prior Art
In one method of seismic exploration an elongated, non-repetitive vibratory signal or wave train is injected into the earth. The injected seismic signal is non-repetitive or random during a time interval which is at least as long as the travel time of a reflection wave along the longest travel path of interest. In normal practice the vibratory signal is therefore at least as long as the travel time of the seismic waves to the deepest reflection to be mapped. The most widely used non-repetitive vibratory signal is one whose frequency changes with time from some predetermined beginning frequency to a predetermined ending frequency. These beginning and ending frequencies are selected to produce a desired pulse shape after processing. This signal, commonly called a pilot signal or control signal, may be generated in any one of several known ways including signal generators or the use of prerecorded magnetic tape.
The pilot signal is assumed to be a replica of the seismic signal injected into the earth. In reality, it is only a replica of the signal which controls the vibrating apparatus. There are a number of factors which tend to distort the injected signal. One of the factors being the operational characteristics of the vibrator itself. The other being the media to which the vibrator is coupled.
In seismic exploration operations which utilize vibrators the relatively long sweep signals give rise to received signals in which reflections from successive interfaces of geological formations overlap. Consequently, in order to interpret the vibrator-type seismogram in the same manner as conventional dynamite seismogram where the input signal is a short pulse, the long sweeps occuring in the vibrator seismogram must be compressed to short signals. This is done by correlating the final vibrator-type seismogram with the sweep or pilot signal. Because of the non-linear reaction of the media to which the vibrator is coupled and/or because of the non-linearity in the vibrator drive, the emitted signal is not only the proper sweep signal but also contains its harmonics. In correlating the vibrator-type seismogram these harmonics lead to assymetrical secondary lobes which occur with upsweeps before the main pulse and with downsweeps after the main pulse. One method of suppressing even harmonics is suggested in U.S. Pat. No. 3,786,409 describing a technique utilizing inverted stacking, the essential features of which is that only one half of the elementary seimograms are generated with the proper sweep. The other half of the elementary seismogram is generated with a sweep which has the opposite polarity, and simultaneously the polarity of the seismic data at the input of the receiving system is reversed. The seismograms resulting therefrom are vertically stacked.
Another technique for reducing harmonics is described in U.S. Pat. No. 4,042,910. This technique suppresses both odd and even harmonics by repeatedly injecting a sweep signal into the earth and receiving that signal through suitable means with the phase of the generated signal being shifted relative to the prior signal by a fixed phase angle which is a fraction of two pi and transforming the recorded signals by inverting the fixed phase shifts in order to re-phase the recorded series ofd signals prior to adding them together to produce a composite record.
In common in the prior art techniques is the reduction of harmonics distortion by changing the character of successive sweeps of the vibrator and thereafter treating the received data.
It is an object of the present invention to provide a method wherein harmonics of the emitted sweep signal, the output of the vibrator, are suppressed up to any order desired by utilizing a pilot or control signal including a corrective component such that harmonic distortion in successive received signals are significantly suppressed without further need to treat the received signals.